从RSS提要中提取时，NSDate dateFromString返回null

Question

我正在我的应用程序中解析RSS提要并将每个项目放入表格视图单元格中。我抓住的其中一件是pubDate，这是一个如下所示的字符串：

Fri, 23 May 2014 10:00:00 GMT

我使用以下代码，但NSDate继续返回null。但是，如果我手动输入字符串（请参阅注释掉的行）而不是尝试抓取pubDate，它可以正常工作。知道我做错了吗？

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 
{
    ReadingsCell *cell = (ReadingsCell *)[tableView dequeueReusableCellWithIdentifier:@"ReadingsCell"];

    cell.readingLabel.text = [[feeds objectAtIndex:indexPath.row] objectForKey: @"title"];
    dateString = [[feeds objectAtIndex:indexPath.row] objectForKey: @"pubDate"];
    //dateString = @"Mon, 19 May 2014 10:00:00 GMT";
    NSLog(@"dateString: %@", dateString);
    //the above logs out perfectly

    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
    [dateFormat setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]];
    [dateFormat setDateFormat:@"EEE, dd MMM yyyy HH:mm:ss Z"];
    [dateFormat setTimeZone:[NSTimeZone timeZoneWithName:@"UTC"]];

    NSDate *postDate = [[NSDate alloc] init];

    postDate = [dateFormat dateFromString:dateString];

    NSLog(@"pubdate: %@", postDate);
    //this logs out as null

    [dateFormat setDateFormat:@"MMM"];
    NSString *theMonth = [dateFormat stringFromDate:postDate];
    cell.monthLabel.text = theMonth;

    return cell;
}